Low pressure thermoplastic molding materials for fiber-reinforced composite structures

ABSTRACT

Fiber-reinforced molding materials and a related method of manufacture are provided. The fiber-reinforced molding materials include a fiber-reinforced tape that is moldable into complex structures at relatively low pressures while having high fiber concentrations. The fiber-reinforced tape includes a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material. The fiber-reinforced tape can be interwoven into multiple woven panels that are consolidated to form a fiber-reinforced mat. The fiber-reinforced mat is suitable for a wide range of molding applications where high strength and light weight are desired, while accommodating a variety of reinforcing fibers and thermoplastic resins.

BACKGROUND OF THE INVENTION

The present invention relates to fiber-reinforced thermoplastic molding materials for forming composite structures.

Fiber-reinforced molding materials are widely used in the manufacture of composite structures where high material strength and light weight are desired. For example, fiber-reinforced sheets are commonly used in the manufacture of automotive components, watercraft hulls, aircraft structures, piping, sporting equipment, and water tanks. In these and other applications, fiber-reinforced sheets include multiple fibers disposed in a matrix material that, when cured, form a lightweight and dimensionally stable structure adapted to withstand external loads.

Fiber-reinforced sheets generally include either continuous fiber strands or randomly oriented fiber segments. Continuous fiber strands include unidirectional fibers, woven fibers and knitted fibers, and are primarily utilized in molding operations involving only a shallow draw. While the finished structure provides excellent strength and modulus, these molding materials are limited in their ability to be shaped into complex parts. That is, shaping sheet materials including continuous fiber strands into corners or cavities is a tenuous process with little high-production success, primarily attributed to the resistance of the continuous fiber strands to stretch or elongate.

Sheet materials including randomly oriented fiber segments exist primarily for the purpose of complex molding. Processes using these sheet materials include injection molding and compression molding. Both processes use sheet materials including relatively short fiber segments, often less than 1 inch, expensive tooling and high curing pressures. The molding processes can destroy and randomize fiber lengths, which ultimately affect the strength, modulus and dimensional stability of the completed part. Because extremely high pressures are used to quickly mold parts in a shortened cure cycle, fiber orientation is often destroyed and made unpredictable. This can lead to completed parts having poor dimensional stability, strength, and modulus.

SUMMARY OF THE INVENTION

Fiber-reinforced molding materials and a related method of manufacture are provided. The fiber-reinforced molding materials include a fiber-reinforced tape having a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material. The fiber-reinforced tape can be interwoven into multiple woven panels that are consolidated to form a fiber-reinforced mat. The fiber-reinforced mat is moldable into complex structures at low pressures for a wide range of applications where high strength, good dimensional stability, and light weight are desired.

In one aspect of the invention, a method includes providing a fiber-reinforced tape including a plurality of unidirectional fibers disposed within a thermoplastic resin, and perforating the fiber-reinforced tape to separate the plurality of unidirectional fibers into a plurality of aligned discontinuous fiber segments. This method can further include heating and pinch rolling the perforated tape to substantially close the perforations, and taking up the fiber-reinforced tape in a spool. The taken-up fiber-reinforced tape includes a plurality of discontinuous fiber segments aligned in unidirectional columns extending lengthwise along the fiber-reinforced tape.

In another aspect of the invention, a method for forming a fiber-reinforced sheet includes providing a first plurality of fiber-reinforced weft strips, providing a first plurality of fiber-reinforced warp strips, and interweaving the warp strips and the weft strips to form a first woven panel, where the warp strips and the weft strips include a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material. The method can further include layering a second woven panel over the first woven panel, the second woven panel including a second plurality of fiber-reinforced weft strips and a second plurality of fiber-reinforced warp strips, where the warp strips and the weft strips include a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material.

In another aspect of the invention, a method for molding a thermoplastic composite structure is provided. The method includes inserting a multi-layered woven mat into a compression mold having the exterior shape of the composite structure. The multi-layered woven mat can include first and second woven panels each including a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material. The method can additionally include closing the compression mold, applying heat and pressure to the multi-layered woven mat within the compression mold, and removing a cured composite structure from the compression mold. Optional additional steps can include finishing the cured composite structure.

These and other features and advantages of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a perforation process in accordance with an embodiment of the present invention.

FIG. 2 is an illustration of trimming and spooling processes in accordance with an embodiment of the present invention.

FIG. 3 is an illustration of a first perforation pattern.

FIG. 4 is an illustration of a second perforation pattern.

FIG. 5 is an illustration of a third perforation pattern.

FIG. 6 is an illustration of a fourth perforation pattern.

FIG. 7 is an illustration of a fifth perforation pattern.

FIG. 8 is an illustration of a weaving process in accordance with an embodiment of the present invention.

FIG. 9 is an illustration of a consolidation of multiple woven panels.

FIG. 10 is an illustration of a compression molding process in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE CURRENT EMBODIMENTS

The invention as contemplated and disclosed herein includes a fiber-reinforced tape and a related method of manufacture. As set forth below, the fiber-reinforced tape is moldable into complex structures at low pressures for a wide range of applications where high strength, dimensional stability, and light weight are desired.

With reference to FIGS. 1-2, the method generally includes providing a unidirectional tape 20 having continuous fibers within a thermoplastic resin. The unidirectional tape 20 can be formed by introducing continuous dry reinforcing fibers into a molten resin, pulling the reinforcing fibers through an impregnation process to fully wet out the fibers and to spread the fibers into a desired dimension. The molten tape is then cooled into a rigid tape while controlling the desired tape width and tape thickness. The resulting tape includes a plurality of collimated bundles of fibers or ‘tows’ and having a desired fiber to resin ratio. The fibers can include any fiber adapted to strengthen the unidirectional tape, including for example fiberglass, aramid, or carbon. The resin can include a thermoplastic resin, including for example polyamide, polyethylene terephtalate, polyphenylene sulphide, polybutylene terephthalate, polysulfone, polycarbonate and combinations thereof. In one embodiment, the fibers are between about 20% and about 80% of the weight of the tape, further optionally between about 60% and about 65% by weight of the tape. Other fibers to resin ratios can also be used as desired, including concentrations within or outside of these ranges.

The continuous fibers are then separated into aligned discontinuous fiber segments in a perforation operation. The perforation operation can be accomplished in-situ with the tape making process or as a separate procedure. As also shown in FIG. 1, the perforation operation includes drawing the unidirectional tape 20 through a perforator 22 to produce a specified and repeating pattern of slits or holes through the entire thickness of the tape 20. The perforation operation can be accomplished by belt or roller tooling. In the present embodiment, the tape is drawn through belt tooling at a rate of about 40 feet per minute to about 150 feet per minute. Where the perforation operation is performed in situ, roller speed is synchronized with the tape manufacturing line rate. The perforation operation can produce a slit, producing no waste, or a punch, producing waste. Where the perforation operation produces a punch, the resulting waste can be collected by vacuum and held in storage. The edges of the tape are generally not perforated, leaving an intact edge which is subsequently removed in a trimming operation as discussed below in connection with FIG. 2.

More particularly, the perforator 22 includes an upper endless belt 24 and a lower endless belt 26 to draw the unidirectional tape 20 therethrough. The upper endless belt 24 includes a plurality of teeth 28 in alignment with a plurality of recesses 30 in the lower endless belt 26. Each belt 24, 26 is trained about a drive pulley 32 and an idler pulley 34, such that the tensioned portion of each belt 24, 26 intercepts the unidirectional tape 20. As the tape 20 enters the perforator 22, discriminate lengths are cut or punched into the continuous fibers to produce multiple discontinuous fiber segments. The length of each discontinuous fiber segment can be dependent on the desired modulus and the desired formability of the finished tape. In the present embodiment, the perforations occur with a frequency of between about 0.5 inches to about 1.0 inch, such that the continuous fibers are separated into discontinuous fiber segments having a length between about 0.5 inches to about 1.0 inch. In other embodiments the discontinuous fiber segments can have a length outside of the above exemplary range. The perforations are generally laterally elongate, being perpendicular to the direction of tape travel. The perforations terminate a predetermined distance from the lateral edges of the tape, optionally to within about 0.03125 inches from the lateral edges.

As also shown in FIG. 1, the perforated tape 20 is heated to above the molten temperature of the thermoplastic resin and immediately cooled and compressed to heal the thermoplastic resin. In particular, pinch rollers 36 apply pressure to re-unify the perforations in the thermoplastic resin while additional rollers 38 cool the re-unified tape while also correcting surface finish and tape thickness. The reunified tape 20 includes an upper surface and a lower surface, each surface being continuous and substantially free from surface discontinuities such as holes or slits.

As shown in FIG. 2, the reunified tape 20 progresses through a trimming operation to remove the lateral edges of the tape 20. In the trimming operation, first and second trimming wheels 40, 42 create respective slit lines along the length of the reunified tape 20, the slit lines intersecting brakes between discontinuous fiber segments. In the present embodiment, the trimming wheels remove about 0.4 inches from the lateral edges of the reunified tape 20, while in other embodiments a greater or lesser amount can be removed. The excess lateral edge portions 43 are taken up by winding spools 45. The now-finished tape 20 is drawn through pull-rollers and taken up into spools 44.

As noted above, the perforation operation includes separating continuous fibers into discontinuous fiber segments according to a repeating pattern of perforations. As shown in FIG. 3, for example, the perforation pattern includes a first row 50 of five perforations 51 and a second row 52 of five perforations 53, where the first and second rows 50, 52 are laterally offset from each other. Each perforation 51, 53 is repeated at 0.5 inch intervals, such that no single discontinuous fiber segment is longer than 0.5inches. Adjacent groups of discontinuous fiber segments 54, 56 are longitudinally offset from each other, such that no single perforation or break 51, 53 in the continuous fiber segments extends the width of the tape 20. The perforation pattern shown in FIG. 4 is similar to the perforation pattern of FIG. 3, except that six perforations 51, 53 extend across the width of the tape 20. The perforation pattern shown in FIG. 5 is also similar to the perforation pattern of FIG. 3, except that ten perforations 51, 53 extend across the width of the tape 20. The perforation pattern of FIG. 6 includes a first row of fifteen perforations 51 and a second row of fifteen perforations 53, where the first and second rows partially overlap each other. The perforation pattern of FIG. 7 includes three rows 50, 52, 60 of perforations 51, 53, 55 repeated at 0.5 inch intervals. Each row includes seven perforations, and each is row is laterally offset from the adjacent row of perforations. This pattern of three rows of perforations is then repeated in columns along the length of the tape in the perforation operation. Accordingly, adjacent discontinuous fiber segments are longitudinally offset from each other in some embodiments, while in other embodiments adjacent groups of discontinuous fiber segments are longitudinally offset from each other.

To reiterate, the finished fiber-reinforced tape 20 generally includes a plurality of discontinuous fiber segments extending unidirectionally in longitudinal columns within a thermoplastic matrix material. The fiber-reinforced tape 20 is substantially free of fibers that are angled relative to the tape longitudinal axis, and in particular, substantially free of fibers oriented at angle of greater than 5 degrees relative to the tape longitudinal axis. The fiber-reinforced tape 20 is substantially free of continuous fibers, with each fiber segment being less than about 5 inches, further optionally less than about 2 inches, and still further optionally between about 0.5 inches and about 1.0 inches. Adjacent groups of discontinuous fiber segments are longitudinally offset from each other as noted above in connection with FIGS. 3-7. The fiber-reinforced tape 20 can assume a number of dimensions based on the needs of the particular application. In the present embodiment, the finished tape can define a width of at least about 0.125 inches, optionally about 2.0 inches. In addition, the fiber-reinforced tape 20 can define a thickness of between about 0.005 inches and about 0.075 inches, optionally between about 0.14 inches and about 0.028 inches, and further optionally about 0.024 inches. In other embodiments, the fiber-reinforced tape 20 can vary outside of these ranges, however.

Once formed, the fiber-reinforced tape 20 can be used in the assembly of woven panel 70. With reference to FIG. 8, a method for forming a woven panel 70 generally includes providing a first plurality of laterally spaced apart fiber-reinforced weft strips 72, providing a first plurality of laterally spaced apart fiber-reinforced warp strips 74, and interweaving the weft and warp strips 72, 74. Each of the weft and warp strips 72, 74 are formed according to the method set forth above in connection with FIGS. 1-2. In addition, each of the weft and warp strips 72, 74 include a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material. The weft and warp strips 72, 74 are optionally identical to each other, providing a woven panel 70 with a desired overall width and fiber to resin ratio. The woven panel 70 can be used as a single sheet, for example in an insert molding process. The woven panel 70 can also be combined with other woven panels. For example, a second woven panel 76 can include a second plurality of laterally spaced apart fiber-reinforced weft strips 72 and a second plurality of laterally spaced apart fiber-reinforced warp strips 74. The first and second woven panels 70, 76 can be consolidated using a press, a laminator, nip rolls, or a belt press. Each panel includes an alternating orientation of 0° and 90° as perhaps best shown in FIG. 9. A third woven panel 78 is also shown as being consolidated to form a multi-layered mat 80. Each panel within the mat 80 can contain films on the exterior surfaces and/or between woven panels to achieve product variability. For example, a surface film can achieve a paintable or class “A” surface. Films can be placed between the woven panels 70, 76, 78 to increase impact resistance or toughness. These films or layers would normally be destroyed in a high pressure molding situation. In the present embodiment, however, these films and layers can remain intact without deformation due to the lower pressure molding of the multi-layered mat 80.

Once the fiber-reinforced tape 20 has been laid up, woven, or subsequently consolidated into a multi-layered mat 80, the fiber-reinforced tape 20 can be placed in a low-pressure mold 82. As shown in FIG. 10, a heated die 84 (for example hydro-form or low-diameter plug) presses a multi-layered mat 80 into the desired shape. When the mat 80 is heated to the point that the impregnation polymer melts, the embedded discontinuous fiber segments slip, allowing the fiber segments to move in a discriminate and controlled manner. Since the multi-layered mat 80 is under relatively low pressure, each fiber segment is constrained by the adjacent fiber segments, and is only allowed to move in the direction it is oriented. This allows each layer of the multi-layered woven mat 80 to expand orthogonally relative to the adjacent layer. As the fiber segments give, they pull apart from each other, allowing the molding material to move into cavities or corners within the mold cavity. Warpage is minimized because the fiber segments are not free to move in any direction, but are confined in the direction of fiber orientation.

The above molding process can be customized to meet needs of a given application. Selection of the thermoplastic matrix and the reinforcing fibers can influence the physical properties of the composite structure. For example, temperature resistance, solvent resistance, and impact resistance can be tailored into the fiber-reinforced tape. In addition, the fiber segment length and fiber to resin ratio can influence the mechanical properties of the composite structure, including the strength and the modulus. As a result, the fiber-reinforced tape of the present invention can provide composite part designers with a range of material options not otherwise available in the manufacture of complex composite structures.

The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. Any reference to elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular. 

1. A method for forming a fiber-reinforced tape comprising: providing a unidirectional tape including a plurality of continuous fibers disposed in a thermoplastic resin; and perforating the unidirectional tape to separate the plurality of continuous fibers into a plurality of discontinuous fiber segments.
 2. The method according to claim 1 wherein adjacent ones of the plurality of discontinuous fiber segments are longitudinally offset from each other.
 3. The method according to claim 1 wherein the unidirectional tape includes an upper surface and a lower surface, each perforation extending from the upper surface to the lower surface of the unidirectional tape.
 4. The method according to claim 1 wherein the thermoplastic resin defines a molten temperature, the method further including heating the perforated unidirectional tape to above the molten temperature of the thermoplastic resin.
 5. The method according to claim 4 further including pinch rolling the heated thermoplastic resin to substantially close the perforations in the unidirectional tape.
 6. The method according to claim 5 further including taking-up the unidirectional tape into a spool.
 7. The method according to claim 1 wherein the plurality of unidirectional fibers includes at least one of fiberglass strands, aramid strands, and carbon fiber strands.
 8. A fiber-reinforced tape comprising: a thermoplastic matrix material; and a plurality of aligned discontinuous fiber segments extending unidirectionally within the thermoplastic matrix material.
 9. The fiber-reinforced tape of claim 8 wherein laterally adjacent ones of the plurality of discontinuous fiber segments are longitudinally offset from each other.
 10. The fiber-reinforced tape of claim 8 wherein laterally adjacent groups of the plurality of discontinuous fiber segments are longitudinally offset from each other.
 11. The fiber-reinforced tape of claim 8 wherein the thermoplastic matrix material is selected from the group consisting of polyamide, polyethylene terephtalate, polyphenylene sulphide, polybutylene terephthalate, polysulfone, and polycarbonate.
 12. The fiber-reinforced tape of claim 8 wherein the plurality of discontinuous fiber segments include at least one of fiberglass, aramid and carbon fiber.
 13. The fiber-reinforced tape of claim 8 wherein the plurality of discontinuous fiber segments are between about 20% and 80% by weight of the fiber-reinforced tape.
 14. The fiber-reinforced tape of claim 8 wherein the fiber-reinforced tape defines a width of at least about 0.125 inches and a thickness of between about 0.005 inches and 0.075 inches.
 15. A method for forming a thermoplastic composite structure, comprising: providing a first plurality of spaced apart fiber-reinforced weft strips; providing a first plurality of spaced apart fiber-reinforced warp strips; interweaving the first plurality of weft strips and the first plurality of warp strips to form a first woven panel, wherein the first plurality of weft strips and the first plurality of warp strips each include a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material.
 16. The method according to claim 15 further including: providing a second plurality of spaced apart fiber-reinforced weft strips; providing a second plurality of spaced apart fiber-reinforced warp strips; interweaving the second plurality of weft strips and the second plurality of warp strips to form a second woven panel, wherein the second plurality of weft strips and the second plurality of warp strips each include a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material; and layering the second woven panel over the first woven panel to form a multi-layered mat.
 17. The method according to claim 16 further including: inserting the multi-layered mat into a mold; applying heat and pressure to the multi-layered mat in the mold; and removing a finished composite structure from the mold.
 18. A woven panel comprising: a plurality of spaced apart fiber-reinforced weft strips; and a plurality of spaced apart fiber-reinforced warp strips being interwoven with the plurality of weft strips at a plurality of junctions, wherein the weft strips and the warp strips include a plurality of discontinuous fiber segments extending unidirectionally within a thermoplastic matrix material.
 19. The woven panel of claim 18 wherein laterally adjacent ones of the plurality of discontinuous fibers are longitudinally offset from each other.
 20. The woven panel of claim 18 wherein laterally adjacent groups of the plurality of discontinuous fibers are longitudinally offset from each other.
 21. The woven panel of claim 18 wherein the thermoplastic matrix material is selected from the group consisting of polyamide, polyethylene terephtalate, polyphenylene sulphide, polybutylene terephthalate, polysulfone, and polycarbonate.
 22. The woven panel of claim 18 wherein the plurality of discontinuous fibers includes at least one of fiberglass, aramid, and carbon fiber.
 23. The woven panel of claim 18 wherein the plurality of discontinuous fibers are between about 20% and 80% by weight of the woven panel. 